It is well known that refined petroleum oils generally exhibit substantial changes in viscosity with temperature. The viscosity index ("V.I.") is a measure of the slope of the temperature-viscosity curve. It is preferred that a lubricating oil, e.g., automobile lubricating oil, exhibit a "flat" V.I. curve. The desired V.I. characteristic is generally achieved by adding oil soluble polymers to oil. For many years the preferred polymer additive was polyisobutylene.
Recently, specialty ethylene-propylene copolymers have been developed and are now widely used as V.I. improvers. Since lubricating oils are used in a wide range of applications, the market requires a variety of grades of such polymers having differing degrees of "thickening effect" so as to permit the formulation of lubricating oils having different viscosities and "Shear stability indicies". Such polymer grades may be prepared by direct synthesis, the molecular weight grades being determined by the polymerization process, or the different molecular weight grades can be produced by degradation of an ethylene-propylene copolymer so as to produce lower molecular weight fractions.
The patent literature is replete with many publications dealing with ethylene ter-and tetrapolymers containing one or more types of dienes introduced for a variety of reasons including a means for introducing unsaturation, thereby providing a means for crosslinking the polymer.
In the case of viscosity index improvers, crosslinking is neither a necessary nor desirable characteristic of the polymer. Illustrative of patents dealing with unsaturated ethylene ter-and tetrapolymers is U.S. Pat. No. 3,790,480. Polymers of ethylene, C.sub.3 -C.sub.18 higher alpha olefins and two classes of dienes are taught, the dienes having double bonds of the same or different polymerizability. In one class of dienes represented by 1,4-hexadiene, only one of the double bonds is readily polymerizable by the catalyst used. In another class of which 2,5-norbornadiene is representative, both double bonds are polymerizable utilizing the polymerization process of the patent. It is taught that the preferred viscosity indexes improvers are ethylene tetrapolymers wherein both classes of double bonds are used.
Presumably, superior properties are achieved because use of a diene with two active double bonds results in long chain branching with a concomitant increase in bulk viscosity of the polymer without any significant increase in intrinsic viscosity or thickening efficiency. Increased bulk viscosity facilitates the manufacture and storage of the polymer. The catalyst used for polymerization is a Ziegler type catalyst. Both double bonds of the 2,5-norbornadiene are polymerizable by the Ziegler catalyst. The other diene, 1-4 hexadiene, however, has only one Zeigler catalyst polymerizable double bond. Hence, the polymers include a minor amount of unsaturation.
Unsaturation in a polymeric viscosity index improving oil additive is generally undesirable since the unsaturated moiety introduces a site through which chemical reactions can occur under the conditions of use of the lubricating oil. Such reactions are undesirable since they cause changes in the viscosity of the lubricating oil. On the other hand, branched saturated ethylene tri or tetra polymers have desirable properties as viscosity modifiers.